This invention relates to travelling-wave tube amplifiers, and more specifically relates to the use of a rejective type filter for connecting a helix type tube to its output circuit.
Helix type travelling-wave tubes generally have an excellent frequency response over a broad band, and are frequently used as power amplifier tubes in various types of AM and FM communication systems. In such travelling-wave tubes, there should be broad band impedance matching between the input and output sections of the tube and their respective external circuits. To meet this requirement, a coupling structure is employed at the input and output sections which directly connects the helix to the inner conductor of a coaxial feeder terminal. Travelling-wave tubes having this structure are disclosed, for example, in an article entitled THE DEVELOPMENT OF SATELLITE TRAVELLING-WAVE TUBES, published in the Journal of the British Interplanetary Society, Vol. 26, pp. 521-532 (1973). Since a travelling-wave tube is a non-linear amplifier, higher harmonic components are inevitably generated especially when it is used as an amplification-saturated amplifier for a frequency-modulated carrier wave. Therefore, in a microwave transmitter, a band-pass or low-pass filter is provided at the most appropriate point between the travelling-wave tube and its antenna to prevent interference due to the higher harmonics and to reduce radiation of the harmful higher harmonics. Such circuits are disclosed, for example, in an article by E. J. Drazy et al. entitled NETWORKS in The Bell System Technical Journal, September 1968 issue, Vol. 47, No. 7, pp. 1397-1422. While higher harmonic components can be entirely absorbed by a band-pass or low-pass filter of the perfectly absorptive type, these filters are large and expensive. Reflective type filters are commonly used, but a reflective type filter can cause transmission distortion depending upon the distance between the filter and the attenuator provided in the slow-wave structure of the travelling-wave tube, and further depending upon the relation between these filters and non-linear amplification characteristics of the travelling-wave tube.